Physical Geology lecture outline - Basic building blocks of rocks - from atoms to minerals.

Heirarchal organization schemes (one potential framework for understanding and learning):

Review of atomic architecture:

What is the elemental make-up of earth's crust?

Above is a diagram of the basic building block (ionic group) of the majority of minerals that compose the earth. Note that sometimes an Al ion can substitute for the Si in the center since they have a somewhat similar size and charge.

What determined the chemical make-up of the earth?

Definition of a mineral: solid/crystalline (regular atomic architecture), inorganic, naturally occurring, fixed or limited range of chemical composition.

Is snow made up of minerals? Is coal made up of minerals? Is volcanic glass made up of minerals?

What are diagnostic properties for hand specimen identification?

Photograph taked from Smithsonian Institute of a model of a halite latic structure showing the regularly repeating character of ion arrangement.

Photograph of unusually large and clear calcite specimen on exhibit at the Smithsonian Institute in Wasahington D.C., showing the rhomb form where the sides (cleavage planes) are at 75 degrees to each other. Also note how the word calcite behind the crystal appears twice. This is because of the ways that light is bent (refracted) and split as it travels through calcite.

Atomic architecture and the relation to hand specimen diagnostic properties:


Above is depicted aportion of a sheet silicate architecture. Each triangle represents a silicate tetrahedral ionic group. They are bound together by sharing oxygen at corners (marked as red fuzzy dots). The blue dot represents the remaining free oxygen for each tetrahedra, all facing upwards. The architecture is that of a an array of three sided pyramids sitting flat on a surface. Now the question is - how are the sheets bound to each other?

Below are the chemical formulas for two common sheet silicates (micas). Which part is the silicate sheet, and which is the glue?

muscovite K Al2 (OH)2 (Al Si3O10)
biotite K
2 (Mg,Fe)2 (OH)2 (Al Si3O10)

Two images from USGS web site that show muscovite (left) and muscovite and biotite (right). Note the one good cleavage reflecting the sheet geometry of this mineral.

Rhyme or reason to the various types of silicate minerals? One organizing factor is the number of tetrahedral oxygen that are shared.

We will see this series of minerals, from less sharing to more sharing of the oxygens, again! This represents something fundamental.

Some sites that provide similar information to the above on silicate minerals as defined by how the silicate tetrahedra are linked.

Solid substitution series in minerals (remember that "limited range of chemical composition" in the definition a mineral - this is why).

What determines the exact composition of an individual mineral?

Information in mineral textures:

Microscope image of a small fluid inclusion in a mineral, with internal minerals and a vapor bubble that grew out of the trapped fluid as it cooled. The contents of the fluid inclusion tell us a lot about the fluids that have moved through and formed minerals. Image from USGS site:

Non-silicate mineral groups and their economic and environmental importance:

Image to right is of one form of hematite, a common iron oxide. Image source USGS site:

Biomineralization - mineral growth influenced by biologic activity. Can you think of any examples? Geobiology is a relatively new sicence that has developed that looks at the interplay between biologic processes (especially microbial) and geologic processes.

Sample of flourite from Smithsonian Institute in Washington, D.C. showing excellent cubic crystal form (but with a different, octahedral, cleavage). Flourite comes in multiple colors and is often zoned, as can be seen in the cross section of the grain on the very left, with a yellow interior. The zoning reflects evolving conditions as the crystal grew. Flourite usually forms from precipitation in hydrothermal fluids circulating in cracks in the earth.

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